1. Field of the Invention
The present invention generally relates to a system and method for measuring properties of traveling webs of sheet material during manufacture and, more particularly, to a system and method for providing measurements such as dry basis weight of fibrous sheet materials.
2. State of the Art
In the art of making sheet materials, an important quality control measure is basis weight. "Basis weight" is normally defined as the weight per unit area of sheet material and is usually stated in units of grams per square inch. A related measure that is particulary useful during the manufacture of fibrous sheet materials, such as paper and cardboard, is known as "dry basis weight"; it refers to weight per unit area of sheet material excluding moisture. In the art of paper-making, dry basis weight is equivalent to the weight of dry material, primarily fibers, comprising a given area of a paper sheet.
It is well known that dry basis weight can be determined by laboratory tests but that such tests have several inherent drawbacks. One shortcoming, for example, is that substantial time is required for sample acquisition and analysis; during that time, substantial quantities of sheet material may be produced and production conditions may change. Another drawback of laboratory tests is that samples obtained for testing may not completely represent sheet material that is produced; this is because samples, for practical reasons, are often obtained only at the end of a sheet roll and, therefore, may not be representative of paper quality at intermediate locations.
It is also known that properties of sheet material can be detected by directing a beam of light or other radiation of known intensity against a surface of a sheet and measuring the radiation absorbed by the sheet. Generally speaking, the amount of absorbed radiation at a particular wave length is a function of the composition of the sheet material in accordance with Beer's law; for example, infrared light having a wavelength of about 1.5 microns is preferentially absorbed by cellulose fibers.
Various devices have been proposed for making on-line parametric measurements of properties of traveling webs of sheet material. (The term "-on line" refers to measurements that are made on a sheet-making machine while the machine is operating; the phrase "parametric measurements" refers to measurements of physical properties whose values determine characteristics and qualities of the sheet material.) Thus, parametric measurements of paper sheet and similar fibrous materials include basis weight, dry basis weight, moisture content, thickness, and transmissivity. In practice, accurate online parametric measurements during paper-making processes are especially difficult to make. The difficulties arise, in part, because modern papermaking machines are large and operate at high speeds; for example, conventional paper-making machines can produce sheets which are 100 to 400 inches wide at the rate of about 20 to 100 feet per second. Furthermore, on-line measurements are problematical in many paper-making factories because of severe environmental conditions; for example, the environment around a paper-making machine may include a high concentration of wet pulp and a humid atmosphere including water droplets and air-borne particles of sulfuric acid or alkalies
To provide on-line parametric measurements of paper and other fibrous sheet materials, workers in the art have proposed various sensors that periodically traverse traveling webs of sheet material. (In the sheet-making art, the direction of sheet travel is known as the "machine direction" and the direction normal to machine direction is known as the "cross direction"; thus, a sensor that moves transversely of a traveling sheet can be said to scan in the cross direction.) For example, U.S. Pat Nos. 3,641,349; 3,681,595; 3,757,122; and 3,886,036 assigned to Measure Corporation discuss basis weight gauges of the scanning type. Also, U.S. Pat. No. 4,289,964, assigned to Intec Corporation, suggests that beta ray gauges can scan slowly across a traveling web in the cross direction to determine basis weight. Further, the Intec patent suggests that beta ray gauges can be augmented with laser, infrared, or ultraviolet light sensors to indicate transmissive qualities across the web; the augmenting sensors are desirable because beta ray sources usually require relatively long periods to generate statistically sufficient numbers of beta rays for measurement purposes.
Despite numerous advantages of scanning gauges in sheet-making operations, such gauges can have limitations. For example, when scanning gauges provide sample measurements infrequently, the measurements may not be optimal for reliable control purposes. This is because a relatively large number of measurements is required in many control situations to provide statistical confidence that a process change is required. In other words, responsive and accurate control often is not always possible with slow-moving scanning gauges. In view of such drawbacks of some scanning devices, it has been proposed to mount fixed sensors at locations distributed along the cross-direction of a traveling web of material. In particular, U.S. Pat. No. 3,806,730 suggests a measuring device that includes a set of radiation emitting tubes mounted to distribute light incident upon the surface of a moving web in the cross-direction. According to the patent, the light tubes are rectangular aluminum pipes that have bright interior surfaces and are filled with inert gas. A second set of similar tubes is positioned to receive light transmitted through the web and to carry that light to a detector. The device described in the patent is said to be useful for measuring parameters such as basis weight and moisture content of paper sheet materials.
As further background to the present invention, it is useful to describe a typical paper-making process. Generally speaking, a paper making process begins when a slurry of fibers and water, called "raw stock", is spread onto a supporting wire mesh from a reservoir called a "head box" through so-called "slice-lip openings." The wire mesh supports the fibers while allowing substantial drainage. To uniformly spread raw stock onto a wire mesh, it is customary to employ gate-like devices mounted next to one another in the cross direction at the slice lip openings. After a wet web is formed on a wire mesh, the web is passed through a press section to express water from the web. Then, the web is passed through a dryer section where water is evaporated from the web. After the dryer section, the web passes through calendar rollers and then usually through a scanner and onto a reel. The portion of a paper-making process prior to a dryer is often referred to as the "wet end" of the process, and the subsequent portion is called the "dry end". It can be appreciated that measurements at the wet end are highly desirable, because such measurements can permit control early in the paper production process and, thus, can minimize wastage by indicating needed process changes before substantial quantities of substandard paper are produced. On the other hand, wet end measurements are difficult to make because of the high water content (usually about 50%) of paper webs and frequently severe environmental conditions. Because of the environment, on-line measuring devices on sheet-making machines often share repair and maintenance problems with the host machines. However, since sheet-making machines often must operate continuously for extended periods, it is of practical importance that repair and maintenance of on-line measuring devices on such machines can be accomplished without causing down-time to the sheet-making machines.